Plasma panel faceplate comprising uv radiation re-scattering means

ABSTRACT

The invention relates to a faceplate comprising a dielectric layer and a protection layer. According to the invention, in order to re-scatter the UV radiation, the interface between the dielectric layer and the protection layer is structured such that it has an average roughness, which is included in the wavelength domain of said radiation, of between 130 and 200 nm in particular. Such re-scattering means are significantly more economical and effective than previous means. The aforementioned roughness can be obtained by performing an abrasion operation on the surface of the dielectric layer.

The invention relates to a plasma display comprising, with reference toFIG. 1:

-   -   a first panel 1 comprising at least a first array Y of        electrodes (not shown) that is coated with a dielectric layer 3        and with a protective and secondary-electron-emitting layer 4,    -   a second array Y′ of electrodes (not shown),    -   a second panel 2 leaving between it and the first panel a space        containing a discharge gas, said space being divided into a        two-dimensional matrix of discharge regions 5,    -   each discharge region 5 being positioned between the electrodes        of the first array and those of the second array and having        walls partly covered with a layer 6 of a phosphor suitable for        emitting visible light when excited by the radiation from a        discharge in this region and    -   the first panel comprising means for backscattering the        discharge radiation onto the phosphors of the corresponding        regions, in this case a scattering layer 9.

The second array of electrodes is generally placed on the first panel insuch a way that, in operation, most of the discharges arise between twoelectrodes of the same panel and are termed coplanar electrodes. Neitherof the two arrays of coplanar electrodes Y, Y′ has been shown in FIG. 1because this represents a cross section made in a plane passing betweenthese electrodes. In general, the second panel includes a third array Xof electrodes that serves for addressing or activating the dischargeregions of the display before what are called the sustain periods.

The dielectric layer 3 is designed to achieve a memory effect so as tobe able, after activation of a discharge region, to sustain a successionof discharges by applying suitable voltage pulses between the electrodesof the first array Y and those of the second array Y′.

The protective and secondary-electron-emitting layer 4 is used toprotect the dielectric layer from bombardment by the ions coming fromthe discharge plasma and is also capable of emitting electrons under theaction of this ion bombardment so as to stabilize the operation of thedisplay.

It is the first panel 1 that is generally transparent to the radiationemitted by the phosphors and that then forms the front image displaypanel; the second panel is therefore the rear panel, which is generallycovered with phosphors in each of the discharge regions.

The discharge regions of the display are, in general and at least inpart, bounded by barrier ribs 7, which form walls for the dischargeregions 5 and serve in general as means of keeping the panels apart; ineach discharge region, the phosphors 6 are generally applied both to therear panel and to the sides of the barrier ribs.

Owing to the nature and the pressure of the gas generally contained inthe space between the panels, the plasma discharges 8 emit ultravioletradiation, indicated in FIG. 1 by the dotted lines.

As shown in the left-hand part of the discharge 8, a first portion ofthis ultraviolet radiation is emitted toward the rear panel 2 and thesides of the ribs 7 and is therefore directly adsorbed by the phosphors6 deposited at that place; the phosphors are then excited and emitvisible radiation that passes through the front panel 2 and thuscontributes to the formation of the image to be displayed: the visibleradiation is indicated in the figure by the solid lines.

As shown in the right-hand part of the discharge 8, a second portion ofthis ultraviolet radiation is emitted toward the front panel 1; becauseof the scattering means with which the front panel is provided, andwhich will be described later, this radiation is backscattered, at leastpartly, into the space between the panels, especially toward thephosphors 6 so as to be converted into visible radiation like the firstportion of the ultraviolet radiation.

It is therefore apparent that the scattering means with which the frontpanel is provided allows a larger portion of the radiation emitted bythe discharges to be converted and substantially increases theluminescence efficiency of the display.

Document EP 1 085 554 teaches how to increase the luminescenceefficiency of plasma displays:

-   -   either by using an ultraviolet radiation reflection layer,        according to the documents cited in paragraph 4 in the above        document; this layer is preferably inserted between the        dielectric layer and the protective and        secondary-electron-emitting layer;    -   or, as shown in FIG. 1, by using a scattering layer 9, deposited        on the protective layer and having a particle size suitable for        obtaining the scattering effect within the wavelength range        corresponding to ultraviolet radiation.

The drawback with the methods for improving the luminescence efficiencythat are described in these documents is that they require an additionallayer, for reflection or scattering, to be added to the front panel;this additional layer adds an additional interface or dioptic systemalong the path of the visible light rays passing through the frontpanel, which impairs transmission of the visible radiation and lessensthe improvements in luminescence efficiency provided by this additionallayer.

Even in the most favorable case, described as a variant in document EP 1085 554, in which the scattering layer has a composition close to thatof the protective layer, for example based on MgO, the process describedin that document for obtaining it is difficult to implement effectively;to achieve the particle size providing the scattering effect, thatdocument teaches depositing it in an aqueous phase, which is prejudicialto the performance of the protective and secondary-electron-emittinglayer, especially to its cathode emission properties under ionbombardment, which are essential for operating stability and lifetime ofthe plasma display.

The object of the invention is to improve the luminescence efficiency ofplasma displays while avoiding these drawbacks.

For this purpose, the subject of the invention is a panel intended toform part of a plasma display and comprising at least a first array ofelectrodes that is coated with a dielectric layer and with a protectiveand secondary-electron-emitting layer,

-   -   said plasma display comprising at least a second array of        electrodes and a second panel leaving between it and the first        panel a space containing a discharge gas,    -   the electrodes of the first array and those of the second array        being arranged so as to leave discharge regions between them and        between the panels, and the walls of these regions being partly        covered with a layer of a phosphor suitable for emitting light        when excited by the radiation from discharges emitted between        the electrodes in these regions,    -   characterized in that the interface between the dielectric layer        and the protective and secondary-electron-emitting layer is        structured so as to have a mean roughness lying within the range        of the wavelengths of said discharge radiation and/or of the        light emitted by said phosphor, especially if this phosphor is a        phosphor emitting in the ultraviolet.

The subject of the invention is also a panel intended to form part of aplasma display and comprising at least one array of electrodes that iscoated with a dielectric layer and with a protective andsecondary-electron-emitting layer, characterized in that the interfacebetween the dielectric layer and the protective layer is structured soas to have a mean roughness of between 130 nm and 400 nm, and preferablybetween 130 and 200 nm.

Owing to the structure of this interface, a large portion of theradiation that is not directly adsorbed and converted by the phosphorsis backscattered toward these phosphors and contributes to theirexcitation; thus, the luminescence efficiency of the display issignificantly improved, at least to a level similar to that of thedisplays described in the abovementioned document EP 1 085 554; oneadvantage of this arrangement is that it is much easier to obtain thanthe scattering or reflection layers described in the prior art, withoutany risk of impairing the performance of the protective andsecondary-electron-emitting layer.

Thus, the panel according to the invention includes means forbackscattering the discharge radiation toward the phosphors; in general,this panel is not coated with phosphors, although such an arrangement isnot excluded.

The mean roughness of the structured interface according to theinvention may be evaluated by using a conventional roughness meter basedon an electromagnetic probe.

Since the protective and secondary-electron-emitting layer is very thin,it generally has the same structure as that of the structured interfaceaccording to the invention, so that it is therefore possible to measurethe roughness of the interface on the surface of the protective layer.

The range of wavelengths of the discharge radiation corresponds to thespectral range comprising more than 90% of the energy emitted by thedischarges.

In most plasma displays, the discharge gas is based on a neon/xenonmixture and the discharges in the displays emit ultraviolet radiation,having two main emission peaks, one at 145 nm and the other at 175 nm;thus, since the wavelength range of the discharge radiation lies withinthe ultraviolet, the mean roughness of said interface is preferablybetween 130 and 200 nm.

Preferably, the protective and secondary-electron-emitting layer isbased on oxides of alkaline earth elements, especially based on magnesia(MgO).

Preferably, the dielectric layer is based on a glassy inorganicmaterial.

The subject of the invention is also a plasma display comprising a panelaccording to the invention and a second panel leaving between it and thefirst panel a space containing a discharge gas, which also includes asecond array of electrodes, the electrodes of the first array and thoseof the second array being arranged so as to leave discharge regionsbetween them and between the panels, and the walls of these regionsbeing partly covered with a layer of phosphor suitable for emittingvisible light when excited by the radiation from the discharges emittedbetween the electrodes in these regions.

Preferably, the first panel according to the invention is the frontpanel of the display; the term “front panel” is understood to mean thatone located on the same side as the person observing the imagesdisplayed by the display; the electrodes placed on this panel are ingeneral transparent. Because the interface between the dielectric layerand the protective layer is structured according to the invention tobackscatter only the radiation emitted by the discharges between thepanels, it absorbs none or very little of the visible light emitted bythe phosphors; this front panel is therefore advantageously transparentto the visible light emitted by the phosphors; it is more transparent tothis light since there are fewer interfaces or dioptic systems to passthrough than in the panels of the prior art that also havedischarge-radiation backscattering or reflection means.

The subject of the invention is also a process that can be used tomanufacture a plasma display panel according to the invention,comprising the deposition of a dielectric layer on at least one array ofelectrodes on this panel and the deposition of a protective andsecondary-electron-emitting layer on the dielectric layer, characterizedin that before said protective layer is deposited, but after thedielectric layer has been deposited, a suitable abrasion operation iscarried out on the surface of the dielectric layer so that the meanroughness of this surface is within the range of wavelengths of thedischarge radiation in the plasma display, in particular so that it isbetween 130 and 400 nm, preferably between 130 and 200 nm.

Such a process is particularly simple and economical; it is preferablyapplicable if the dielectric layer is based on a glassy inorganicmaterial, that is to say based on enamel; such an enamel layer isgenerally obtained by depositing a layer based on a dielectric enamelfrit followed by this being baked under conditions suitable forobtaining a dense layer having a smooth surface; the abrasion operationon this surface is then carried out just after the enamel-baking step;this abrasion operation modifies the surface roughness of the enamel;next, the protective layer, generally based on MgO, is deposited in aconventional manner; as this protective layer is very thin, the layerobtained generally has the same roughness as the surface of the enamellayer.

Preferably, the abrasion operation on the surface of the dielectriclayer is carried out by friction of a plastic encrusted with abrasivepowder against this surface; this is a method commonly used forpolishing or lapping glass surfaces or metallographic specimens; theplastic is preferably a polishing felt, for example based on rigidpolyurethane foam, having open pores on the surface, that can contain orretain abrasive powder particles; plastic pastes incorporating theabrasive powder may also be used.

When the aim is to have a mean roughness between 130 and 200 nm, theparticle diameter of the abrasive powder is preferably between 0.2 and 2μm; this is in practice the size of the abrasive particles suitable forobtaining a dielectric layer surface having a mean roughness between 130and 200 nm.

Preferably, the abrasion operation is carried out dry or in a liquidmedium containing no water; a special felt encrusted with abrasivepowder particles is then used.

By carrying out the operation in the absence of water, any deteriorationof the dielectric layer is thus avoided and the correct cathode emissionperformance of the protective layer is more easily guaranteed, therebyimproving the lifetime of the display.

The invention will be more clearly understood on reading the descriptionthat follows, given by way of non-limiting example and with reference tothe appended figures, in which:

FIG. 1, already described, is a schematic sectional representation of aplasma display cell of the prior art; and

FIG. 2 illustrates, in the same representation, a preferred embodimentof the invention applied to the same type of cell.

To simplify the description and bring out the differences and advantagesafforded by the invention compared to the prior art, identicalreferences will be used for those elements that fulfil the samefunctions.

The description starts, with reference to FIG. 2, with a preferredexample of the process for obtaining a plasma display with a highluminescence efficiency according to the invention, in the case in whichthis display is of the AC type with memory effect; this displaycomprises a transparent front panel 1′ with pairs of coplanar electrodesand a rear panel 2.

The manufacture of the front panel 1′ will firstly be described.

Deposited conventionally on a soda-lime glass plate with the dimensionsof the display to be produced are two arrays Y, Y′ of parallel coplanarand interspersed electrodes in such a way that each electrode of thefirst array is adjacent an electrode of the second array; each pair ofelectrodes thus formed therefore corresponds to a row of pixels of thedisplay; each electrode is, for example, formed from a narrow opaque busfor distributing the discharge current and from a transparent conductingstrip, for example made of ITO (indium tin oxide) deposited along thebus and in contact therewith; in this case, electrodes of one and thesame pair face each other via one side of their respective transparentstrip.

Next, a paste based on a dielectric enamel frit is prepared, this beingdeposited on the arrays of electrodes as a layer of uniform thicknessover the entire active surface of the panel; according to a variant,only the electrodes of the arrays Y, Y′ may be covered; apart from thisenamel frit, the above paste contains a polymer-based organic binderand, in general, a solvent for this binder; after deposition, drying, inorder to evaporate the solvent, and where appropriate crosslinking ofthe organic binder, the enamel layer is baked in order to remove theorganic binder from the layer and to vitrify the enamel so as to obtaina uniform dielectric enamel layer 3′; after baking, the layer obtainedhas a smooth and plane surface which, in this state, would allow theradiation emanating from the discharges to pass through it; thethickness of the dielectric layer is generally between 10 and 50 μm.

The next step is specific to the invention: it consists in modifying thesurface finish of the dielectric layer in order to give this surface theability to scatter the ultraviolet radiation that the discharges willemit, especially between the electrodes of the arrays Y, Y′ in thedisplay when operating.

For this purpose, an abrasion operation is carried out on this surfaceso as to obtain a dielectric surface that is no longer smooth aspreviously, but one having a mean roughness lying within the range ofthe wavelengths of the radiation that will be emitted by the dischargesin the display during operation; conventionally, this range is that ofultraviolet radiation and this operation is carried out so as to givethe dielectric surface a mean roughness of between 130 and 200 nm; thismean roughness is, for example, measured using a roughness meter with anelectromagnetic head, such as an instrument of the DEKTAK brand.

To carry out this abrasion operation, many known methods may be usedsuch as, for example, mechanical lapping using a very fine abrasionpowder.

After baking, the surface of the enamel lends itself better to amechanical lapping operation using a very fine abrasive; it is preferredto use commercially available abrasives with particle sizes between 0.2μm and 2 μm, either as pastes (diamond, alumina, carborundum), or on afelt for dry polishing; more precisely, one of the following methods mayfor example be carried out:

-   -   lapping in a liquid medium with a diamond paste, using a        lubricant, preferably one that is inert and chemically inactive        with respect to the enamel layer; it is preferred to use a heavy        alcohol, for example of the isopropanol type, compatible with        the paste containing the diamond-based abrasive powder; it is        advantageous to avoid the use of water so as to better guarantee        the properties of the MgO-based protective layer to be deposited        on the lapped surface;    -   dry lapping using a special felt containing the abrasive powder,        for example of the “glass paper” type; by thus avoiding the use        of water, the properties of the MgO-based protective layer to be        deposited on the lapped surface are advantageously preserved.

To improve the efficiency and the uniformity of this mechanical lappingoperation, it is preferred to use suitable machines that impart acomplex movement to the felt holder or slurry holder (“satellites”) onthe surface to be lapped; this type of machine is widely used forlapping or polishing glass surfaces.

Without departing from the invention, other mechanical abrasion methodsmay be used, such as blasting the surface with a carrier gas containingabrasive powder (or “sandblasting”); it is also possible to use chemicalabrasion methods, electroerosion methods and chemical-mechanical methodswell known to those skilled in the art of surface treatments.

After this abrasion operation specific to the invention, the dielectriclayer now has a “structured” surface:

-   -   which will no longer allow the radiation coming from the        discharges through it, but will backscatter said radiation        toward the interior of the display and    -   which, like the smooth and plane starting surface, will,        however, allow through the visible radiation emitted by the        phosphors deposited on the rear panel, of which mention will be        made later.

After this abrasion operation, a protective andsecondary-electron-emitting layer 4′, in this case based on MgO, isdeposited in a manner known per se, for example by vacuum evaporation;the thickness of the layer obtained is generally between 0.5 and 1.5 μm.

Since the layer obtained is very thin, it is found that the roughnessand the structure of the surface of the dielectric layer is related tothe external surface of the protective and secondary-electron-emittinglayer.

Using the abovementioned conventional abrasion methods, it is found thatthe structure of the surface of the dielectric layer 3′ at the interfacewith the protective layer 4′ is of the “spatial noise” type, like thestructure of the protective layer itself; such structure is differentthan that of the scattering layers described in the abovementioneddocument EP 1 085 554 that are obtained by precipitation in an aqueousmedium.

The front panel 1′ according to the invention is capable ofbackscattering the ultraviolet radiation but of allowing the visibleradiation through, thanks to the structure of the interface between thedielectric layer 3′ and the protective layer 4′, this structure beingsuitable for giving a mean roughness lying within the range ofwavelengths of the discharge radiation, especially between 130 and 200nm; such backscattering means are much more economical and effectivethan those of the prior art because such a roughness may be obtained bya simple abrasion operation. In addition, because there is no operationto deposit an additional layer specifically for reflecting orbackscattering the UV, the panel obtained has a much higher mechanicalstrength.

With regard to the rear panel 2 of the display according to theinvention, this is produced in a manner known per se for obtaining apanel comprising, on a soda-lime glass plate 12:

-   -   a third array X of electrodes that extends perpendicular to the        direction of the electrodes of the arrays Y, Y′ on the front        panel;    -   an enamel-based dielectric layer 13;    -   an array of barrier ribs 7 suitable for defining the discharge        regions and such that said discharge regions are, after the        panels have been joined together, positioned at the intersection        of the electrodes of the array X and of the pairs of        interspersed electrodes of the arrays Y, Y′ of the first panel;        and    -   phosphor layers 6 deposited on the walls of the discharge        regions thus defined, that is to say both on the bottom of these        regions in contact with the dielectric layer 13 and on the sides        of the ribs 7.

Next, the front panel 1′ is joined to the rear panel 2 so that theelectrodes of the array X of the rear panel 2 intersect the pairs ofelectrodes of the arrays Y, Y′ of the front panel 1′ between the ribs 7;the ribs 7 then serve as means of keeping the panels 1′, 2 spaced apart.

The two panels are sealed together in a manner known per se, the gascontained in the space between the panels 1′ and 2 is pumped out andthis space filled with a discharge gas, generally comprising xenon.

The plasma display according to the invention is then obtained; thestructure, specific to the invention, of the surface of the dielectriclayer 3′ at the interface with the protective layer 4′ allows asubstantial portion of the radiation not directly absorbed and convertedby the phosphors to be recovered, so it can be backscattered towardthese phosphors; thus, the luminous efficiency of the display issignificantly improved, to a level at least similar to that of thedisplays described in the aforementioned document EP 1 085 554, whileavoiding a specific scattering or reflection layer in the front panel ofthe display; advantageously, thanks to the invention, the MgO-basedprotective layer may be very easily shielded from any trace of water,thereby better ensuring the cathode emission properties of this layerand the lifetime of the display.

1. A panel intended to form part of a plasma display and comprising atleast a first array of electrodes that is coated with a dielectric layerand with a protective and secondary-electron-emitting layer, said plasmadisplay comprising at least a second array of electrodes and a secondpanel leaving between it and the first panel a space containing adischarge gas, the electrodes of the first array and those of the secondarray being arranged so as to leave discharge regions between them andbetween the panels, and the walls of these regions being partly coveredwith a layer of a phosphor suitable for emitting light when excited bythe radiation from discharges emitted between the electrodes in theseregions, wherein the interface between the dielectric layer and theprotective layer is structured so as to have a mean roughness lyingwithin the range of the wavelengths of said discharge radiation and/orof the light emitted by said phosphor.
 2. A panel intended to form partof a plasma display and comprising at least one array of electrodes thatis coated with a dielectric layer and with a protective andsecondary-electron-emitting layer, wherein the interface between thedielectric layer and the protective layer is structured so as to have amean roughness of between 130 nm and 400 nm.
 3. The panel as claimed inclaim 1, wherein the protective and secondary-electron-emitting layer isbased on oxides of alkaline earth elements.
 4. The panel as claimed inclaim 3, wherein the dielectric layer is based on a glassy inorganicmaterial.
 5. A plasma display comprising a panel as claimed in claim 1and a second panel leaving between it and the first panel a spacecontaining a discharge gas, which also includes a second array ofelectrodes, the electrodes of the first array and those of the secondarray being arranged so as to leave discharge regions between them andbetween the panels, and the walls of these regions being partly coveredwith a layer of phosphor suitable for emitting visible light whenexcited by the radiation from the discharges emitted between theelectrodes in these regions.
 6. The plasma display as claimed in claim5, wherein said first panel is the front panel of said display.
 7. Aprocess that can be used to manufacture a plasma display panel asclaimed in claim 1, comprising the deposition of a dielectric layer onat least one array of electrodes on this panel and the deposition of aprotective and secondary-electron-emitting layer on the dielectriclayer, wherein before said protective layer is deposited, but after thedielectric layer has been deposited, a suitable abrasion operation iscarried out on the surface of the dielectric layer so that the meanroughness of this surface is within the range of wavelengths of thedischarge radiation in the plasma display.
 8. The process as claimed inclaim 7, wherein the mean roughness of said surface is between 130 nmand 400 nm.
 9. The process as claimed in claim 7, wherein the abrasionoperation on said surface is carried out by friction of a plasticencrusted with abrasion powder against said surface.
 10. The process asclaimed in claim 9, wherein the particle diameter of the abrasive powderis between 0.2 and 2 μm.
 11. The process as claimed in claim 9, whereinthe abrasion operation is carried out dry.
 12. The process as claimed inclaim 9, wherein the abrasion operation is carried out in a liquidmedium containing no water.